Phase locked loop circuitry is a form of frequency feedback control circuitry used extensively to receive and track an incoming signal that may be imbedded in a high ambient noise and that may have been drifting in frequency caused by the transmission medium. Generally the phase locked loop circuitry includes a forward path which includes a frequency modulator for modulating an incoming signal and a low pass filter for filtering out the high frequency sidebands and recovering the transmitted signal and a feedback path with a voltage controlled oscillator to provide the injection or local oscillator frequency to the modulator and frequency error correction.
It is also found that the VCO tracking error in frequency increases as the frequency output of the VCO increases. Also the wider the spectrum of the incoming signals, the greater the VCO error tends to be. The accuracy of signal detection and correction is found to fluctuate widely when the incoming signals are composed of signals of different frequency characteristics, such as, is the case with a voice band signal composite of voice, data and a particular inband tone signal encountered in telecommunication systems.